Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Summary Of Significant Accounting Policies [Line Items]
Unbilled revenues	$ 66.7		$ 66.7		$ 86.6
Franchise fees and gross receipts taxes	29.3	$ 27.8	56.6	$ 55.0
Tampa Electric Company [Member]
Summary Of Significant Accounting Policies [Line Items]
Unbilled revenues	60.3		60.3		$ 49.3
Franchise fees and gross receipts taxes	$ 29.3	$ 27.8	$ 56.6	$ 55.0